Designers, painters, contractors, photographers, and, in general, anyone interested in looking for an accurate color information may find the current range of color measurement tools to be limited, bulky, or lacking in convenience. One concern with color measurement tools is how to effectively calibrate color readings for consistency and accuracy. In some systems, an initial calibration may be performed at a time of manufacture. However, such a system is not capable of verifying consistency or accuracy after the device is initially calibrated.
The primary tools for measuring light (color) are colorimeters and spectrometers. Colorimeters include various types, including but not necessarily limited to RGB sensor based, tristimulus sensor based, and multi filter based colorimeters.
When measuring color, the goal for all of these systems is to convert from a sensed set of values into standard CIE color space values (e.g., XYZ, Lab, xyY), and then further color applications can be applied after this (e.g., paint matching, RGB color coding, etc.).
Spectrometers measure color by separating a measured lightwave into wavelength segments, and then integrating over these segments using standard observer curves to transform from wavelength into the standard CIE color space.
Colorimeters go about solving the same problem via a combination of light filters and/or light sources. In the case of a colorimeter that uses an RGB filters (which sense light across spectrum frequencies that roughly represent red, green, and blue), the colorimeter illuminates an object, and then reads back a sense value from the RGB filters. The values provided by the RGB sensor are not very useful until converted into a standard color space.
Tristimulus sensor based colorimeters work in a similar fashion, but the wavelengths covered by the filters are meant to closely follow those of the standard observer curves. Even with output from the tristimulus sensor mimicking the standard observer curves, some level of calibration/conversion is still needed to accurately represent color values (and also account for device to device differences). This is also the case for spectrometers.
When comparing the difference in the way these types of sensors measure color (or comparing the difference in two colors in general), a color difference standard known as ΔE is most typically used. This is a special form of color space “distance” which has been developed by CIE to mimic the way the human eye perceives color difference. The process for converting from sense values and minimizing the ΔE between sensor readings and a standard color value is where instrument calibration and setup becomes important, and will heavily impact the accuracy of a color measurement tool.
One known problem with colorimeters is that they lack the specificity of a spectrophotometer due to much fewer independent measurable parameters in the optical spectrum (i.e., an RGB colorimeter might only measure R, G, and B, whereas a spectrophotometer can record high resolution reflectance curves over the visible spectrum). Inherent design differences such as optics, stimulation sources, and detector responses make colorimeters produce a different result from a spectrophotometer and from other colorimeters.
Because of the aforementioned specificity (and greater perceived accuracy), spectrophotometer measurements are often used as reference standard. Interestingly, there does exist variation between any two spectrophotometers, and it is a common practice to calibrate a spectrophotometer to match the readings of a given reference spectrophotometer. The calibrated accuracy of this device will then be based on its conformance to the reference device.
More generally, all color measurement devices are judged based on their conformance to some reference device. Therefore, the challenge in the development of a color sensing device is to find a method that allows for the device to align with a given reference device.